Thanks Jon it does look like pressure forced petrol up through the breather pipe. Still don't see how the pressure built up though. Maybe evaporated fuel with nowhere to go increased the pressure in the tank. Have turned the tank right way round now and got covered in a spurt of petrol, though that appears to be all that is coming out now. The petrol cap only seems to be seeping a tiny bit. Just enough to leave a damp ring on a sheet of newspaper so that can't be the cause of a petrol soaked shed floor.
I am annoyed with Haynes for crap advice and intend to tell them so. Just hope the shed doesn't burn down in the meantime.
